The present invention relates to a device for supplying secondary air for purifying exhaust gases discharged from an internal combustion engine.
The exhaust gases discharged from an internal combustion engine contain toxic compounds; that is, CO, HC and NOx. CO and HC are unburned compounds so that secondary air may be charged into the exhaust gases to burn them. For this purpose, air pumps which are driven by the engines are generally used, but in some engines the secondary air is charged by utilizing the pulsation of exhaust gas pressure in the exhaust system. To remove NOx, the exhaust gas recirculation systems have been widely used and may be generally divided into the following two types depending upon the underlying principles:
(1) THE TYPE IN WHICH FIXED RESTRICTION MEANS IS USED FOR CONTROLLING THE VOLUME OF EXHAUST GASES TO BE RECIRCULATED FROM THE EXHAUST PIPE TO THE INTAKE PIPE OF THE ENGINE AS A FUNCTION OF THE BACK PRESSURE (THE EXHAUST GASES TO BE RECIRCULATED BEING CHARGED INTO THE INTAKE PIPE UPSTREAM OF THE THROTTLE VALVE), AND
(2) THE TYPE IN WHICH A VARIABLE RESTRICTION MEANS IS USED FOR CONTROLLING THE VOLUME OF EXHAUST GASES TO BE RECIRCULATED (THE EXHAUST GASES TO BE RECIRCULATED BEING CHARGED INTO THE INTAKE PIPE DOWNSTREAM OF THE THROTTLE VALVE). The basic principle common in both types is that the volume of exhaust gases to be recirculated is made proportional to the volume of intake air.
The quantities of unburned compounds in the exhaust gases are generally in proportion to the volume of intake air so that it follows that the secondary air to be charged into the exhaust system for burning the unburned compounds must be made proportional to the volume of intake air. Therefore when the engine load is high so that the volume of intake air is greater, the secondary air supply must be increased accordingly. On the other hand when the engine load is low so that the volume of intake air is small, the secondary air supply must be decreased accordingly. Therefore the secondary air supply must be controlled depending upon the load even when the engine is running at the constant speed. However, the conventional air pump for supplying the secondary air is driven by the engine so that the discharge is dependent only on the rotational speed of the engine, but is completely independent of the load. In the conventional secondary air supply system of the type utilizing the pulsation of exhaust gases, the secondary air supply is dependent upon the negative pressure produced by the pulsation of the exhaust gas pressure and the number or periodic cycle of negative pressure created. Since these two factors are entirely dependent upon the rotational speed of the engine, the secondary air supply is also dependent upon the rotational speed.
Therefore in the conventional secondary air supply systems, the secondary air supply is adjusted in quantity sufficient to burn the unburned compounds discharged when the engine is running with high load so that the insufficient supply of secondary air may be avoided. As a result of this, the secondary air is supplied in excessive quantities when the engine is running at the same speed but with light load so that the exhaust gas purifying device such as a thermal reactor is overcooled by the excessive secondary air with the resultant poor performance of the purifying device.
When the conventional secondary air supply systems are used together with the exhaust gas recirculation systems, the former adversely affects the latter. In case of the exhaust gas recirculation system of the type (1) above, the volume of the exhaust gases to be recirculated is controlled as a function of the back pressure, which is a function of the volume of intake air. However, when the secondary air is supplied independently of the volume of intake air, the above relationship between the back pressure and the volume of intake air cannot be held any longer. Therefore the volume of the exhaust gases to be recirculated is deviated from the optimum level which must be dependent upon the volume of intake air. Similar phenomenon is observed when the performance of the air pump drops. As a result, the exhaust gases tend to be recirculated in excessive quantities especially when the engine is running under light load so that the engine operation is adversely affected. The same problem is also observed in the exhaust gas recirculation system of the type (2) when combined with the conventional secondary air supply systems.